This invention relates to keyboards, keypads, and the like, and more particularly, to a switch for a touchtype keyboard or keypad, for use with, for example, a personal computer or the like.
In essence, a keyboard for a computer comprises a series of switches connected to a microprocessor that monitors the state of each switch and initiates a specific response to a change in that state.
In a typical keyboard, a key matrix is provided which comprises a grid of circuits underneath the keys, whereby each circuit is broken at the point below a specific key. Pressing the key bridges the gap in the circuit, allowing a tiny amount of current to flow through. The processor monitors the key matrix for signs of continuity at any point on the grid. When it finds a circuit that is closed, it compares the location of that circuit on the key matrix to a character map in its ROM. The character map is basically a comparison chart for the processor that tells it what the key at x, y coordinates in the key matrix represents.
Keyboards rely on switches that cause a change in the current flowing through the circuits in the keyboard. When the key presses the keyswitch against the circuit, there is usually a small amount of vibration between the surfaces, known as bounce. The processor in the keyboard recognizes that this very rapid switching on an off is not caused by repeated pressing of the same key, and filters such tiny fluctuations out of the signal and treats it as a single key press.
Keyboards use a variety of switch technologies, although it is interesting to note that a user tends to like to have some audible and tactile response when typing on the keyboard. Such different technologies include: rubber dome mechanical, capacitive non-mechanical, metal contact mechanical, membrane mechanical and foam element mechanical.
Probably the most popular switch technology in use today is rubber dome. In such keyboards, each key sits over a small, flexible rubber dome with a hard carbon centre.
When the key is pressed, a plunger in the bottom of the key pushes down against the dome. This causes the carbon centre to push down also, until it presses against the hard flat surface beneath the key matrix. As long as the key is held, the carbon centre completes the circuit for that portion of the matrix. When the key is released, the rubber dome springs back to its original shape, forcing the key back up to its rest position.
Rubber dome switch keyboards are inexpensive, have relatively good tactile response and are fairly resistant to spills and corrosion because of the rubber layer covering the key matrix. Membrane switches are very similar in operation to rubber dome keyboards. A membrane keyboard does not, however, have separate keys. Instead, it has a single rubber sheet with bulges for each key, and are often used in devices designed for heavy industrial use or extreme conditions. However, because they offer little or no tactile response and can be somewhat difficult to manipulate, these keyboards are seldom found on normal computer systems. Furthermore, the membrane cannot be used if even one of the bulges is defective, and each pad molding is specific to a particular keyboard design.
Waterproofing a keypad suitable for machine controls by clamping the periphery of the elastomeric pad between the top and bottom shells of the keypad has been proposed in the past. However, such keypads do not have, nor are they intended to have, a touch type feel, and there has been no disclosure or suggestion as to how such proposals could be practically applied to much larger, high volume, touch type keyboards.
Although, as stated above, rubber dome switch keyboards are relatively fairly resistant to spills and corrosion because of the rubber layer covering the key matrix, they are not waterproof, and a keyboard will, therefore malfunction due to a short circuit if liquid enters the keyboard. Nevertheless, the rubber dome keyboards remain one of the most popular types of keyboard because of their relatively good tactile response.
In an attempt to provide a waterproof dome switch keyboard, some arrangements have been proposed. However, such arrangements tend to a have a complex structure, are costly to manufacture and, in any event, are still not completely waterproof, such that liquid can still enter and cause a short circuit.
For example, in the arrangement disclosed in U.S. Pat. No. 4,705,925 use is made of individual domes. Each dome is secured in a keycap housing to facilitate handling and assembly. The base of each dome in an assembly is pressed against the membrane to exclude dust, but by doing so affects the feel of the keys because of air pressure inside the dome. Such sealing does not, in any event, waterproof the keyboard. U.S. Pat. No. 4,021,630 describes a switch for a surgical knife in which a dome is clamped between a housing and the membrane to effect a waterproof seal, but it is not intended to have a touch type feel, and would not be practically suitable for use in a computer keyboard or the like.
It is therefore an object of the present invention to provide a switch for a keyboard, which is completely waterproof, whilst also being relatively simple in construction and inexpensive to manufacture.